1. Field of the Invention
The present invention relates to a magnetic material and, more particularly, to a magnetic material capable of realizing magnetic refrigeration using a relatively low magnetic field in near room temperature region.
2. Description of the Related Art
Presently, a gas compression/expansion cycle is primarily used in refrigeration systems, e.g., refrigerators, freezers, and air-conditioners, closely related to daily human life. However, this gas compression/expansion cycle is environmentally problematic, in terms of the exhaustion of specific freon gases. In addition, substitute freon gases presumably have a bad effect on the environment. From this background, clean and efficient refrigeration technologies causing no environmental problems due to wastage of operating gases are being demanded to put into practical use.
Recently, magnetic refrigeration is being increasingly expected as one such environment-friendly, highly efficient refrigeration technology. Much research and development of the related technologies for use in near room temperature region has been undertaken. Magnetic refrigeration generates low temperatures as follows by applying the magnetocaloric effect (a phenomenon in which when an external magnetic field is changed with respect to a magnetic material while the magnetic material is thermally insulated, the temperature of this magnetic material changes).
The entropy of a magnetic material is changed depending on whether a magnetic field is applied or not, owing to the difference between the degrees of freedom of the electron spin system. With this entropy change, the entropy transfers between the electron spin system and the lattice system. Magnetic refrigeration uses a magnetic material having a large electron spin and produces a large entropy change between the magnetic field applied state and the magnetic field removed state. Using this large entropy change, magnetic refrigeration generates low temperatures.
Note that in a temperature region of 1 K or more, a “magnetic material”usually means a substance which shows magnetism due to an electron spin. On the other hand, in a temperature region of a few mK or less, magnetism due to a nuclear spin becomes relatively large. For example, an ultra low temperature of 27 μK was reportedly generated by using PrNi5. In the following description, a “magnetic material” means a substance showing magnetism due to an electron spin, and a target temperature region is 1 K or more, unless otherwise specified.
In the early 1900s, magnetic refrigeration systems using paramagnetic salts such as Gd2(SO4)3.8H2O and paramagnetic compounds represented by Gd3Ga5O12 (gadolinium gallium garnet “GGG”) were developed. However, these magnetic refrigeration systems using paramagnetic materials were in most cases applied to an low temperature region of 20 K or less for the following reason. That is, as the lattice vibration increases with an increase in temperature, the higher magnetic field is required to reduce the lattice vibration by entropy transferring between electron spin system and lattice system. Therefore, assuming a magnetic field of about 10 tesla which can be obtained using an ordinary type superconducting magnet, temperatures at which magnetic refrigeration can be achieved using paramagnetic materials are limited to the low temperature region of 20 K or less.
On the other hand, to realize magnetic refrigeration at higher temperatures, the research of magnetic refrigeration using the magnetic phase transition of a ferromagnetic material between a paramagnetic state and a ferromagnetic state was extensively made after the 1970s. As a consequence, a large number of magnetic materials containing rare earth elements having a large electron magnetic spin per unit volume have been proposed. Examples are lanthanoide rare earth elements such as Pr, Nd, Dy, Er, Tm, and Gd, rare earth alloy materials containing two or more rare earth elements such as Gd—Y and Gd—Dy, and rare earth intermetallic compounds such as RAI2 (R represents a rare earth element, and this similarly applies to the following description), RNi2, and GdPd.
When any of these ferromagnetic substances are used, an external magnetic field is applied at a temperature close to the ferromagnetic phase transition temperature (Curie temperature; Tc), thereby causing the magnetic phase transition of electron spin system from a paramagnetic state to a ferromagnetic ordered state. Magnetic refrigeration is realized by using the resulting entropy change. Therefore, the applicable temperature range is limited to the vicinity of the ferromagnetic phase transition temperature (Tc) of each magnetic material. However, the magnitude of the external magnetic field need only be the one enough to assisting a magnetic phase transition. A magnetic field of this magnitude can be well generated in a temperature region much higher than 20 K.
In 1974, Brown (U.S.A.) achieved magnetic refrigeration at room temperature for the first time, by using a ferromagnetic substance Gd plate having a ferromagnetic phase transition temperature (Tc) of about 294 K. Unfortunately, although the refrigeration cycle was continuously operated in the experiment, there were some problems such as heat transfer in a refrigeration cycle, because an integral Gd plate was used in the range of the hot end to the cold end.
Magnetic refrigeration in the range of intermediate temperatures much higher than 20 K to room temperature has a substantial problem. That is, lattice vibration becomes large as the temperature rises, so, in a temperature region of 100 to 150 K or more the lattice system entropy becomes large compared with the magnetic entropy of the electron spin system. Accordingly, even when the entropy is exchanged between the electron spin system and the lattice system by changing the magnitude of the external magnetic field, the magnetocaloric effect, i.e., a temperature reduction (ΔTad) of the magnetic substance is small.
In 1982, Barclay (U.S.A.) attempted to use the lattice entropy positively that had been regarded as an interference to magnetic refrigeration in the range of intermediate temperatures to room temperature (or a temperature range in which the lattice entropy is large relative to the magnetic entropy), and proposed a method of refrigeration (U.S. Pat. No. 4,332,135) in which a magnetic material is used, in addition to magnetic refrigeration by the magnetocaloric effect, as the regenerator for storing coldness generated by the refrigeration. This magnetic refrigeration method is called AMR (“Active Magnetic Refrigeration”).
In 1997, Zimm, Gschneidner, and Pecharsky of the U.S.A. built a prototype AMR machine using a packed column filled with fine spherical Gd, and succeeded in a continuous steady-state operation of the magnetic refrigeration cycle at room temperature (“Advances in Cryogenic Engineering”, Vol. 43, 1998). According to this reference, refrigeration at about 30° C. was accomplished by changing the magnitude of the external magnetic field from 0 to 5 tesla by using a superconducting magnet at room temperature. When the refrigerating temperature difference (T) between the hot end and the cold end was 13° C., a very high refrigeration efficiency (COP=15; excluding the power input to the magnetic field generating means) was reportedly obtained. Note that the refrigeration efficiency (COP) of a gas compression/expansion cycle (e.g., a household refrigerator) using conventional freon is about 1 to 3.
In addition to the above-mentioned technical demonstration of the AMR-cycle magnetic refrigeration system using Gd, Pecharsky and Gschneidner of the U.S.A. developed a Gd5(Ge,Si)4-based material as a magnetic material from which a very large entropy change can be obtained at room temperature (U.S. Pat. No. 5,743,095). For example, Gd5(Ge0.5Si0.5)4 shows an entropy change (ΔS) of about 20 J/(kg. K) when the magnitude of the external magnetic field is changed from 0 to 5 tesla at about 277 K, and shows an entropy change (ΔS) of about 15 J/(kg. K) when the magnitude of the external magnetic field is changed from 0 to 2 tesla. That is, a large entropy change twice or more that of Gd is observed near room temperature.
Unfortunately, in the experiments conducted by Zimm, Gschneidner, and Pecharsky described above, a superconducting magnet was used to apply a large external magnetic field of about 2 to 5 tesla to Gd as a magnetic material for magnetic refrigeration. Since under the present conditions a cryogenic environment at about 10 K is necessary to operate a superconducting magnet, the system increases in size. In addition, when a superconducting magnet is to be used, it is necessary to use a freezing medium such as liquid helium or a refrigerator for cryogenic generation. It is impractical to apply a system like this to ordinary purposes such as refrigeration and air-conditioning.
A heavy duty electromagnet is another means for generating a large magnetic field, other than a superconducting magnet. When this electromagnet is to be used, however, a large input current and water cooling against Joule heating are necessary. This makes the system larger and also increases the operation cost. Accordingly, similar to the case of a superconducting magnet, it is impractical to apply a system using an electromagnet for usual purposes.
A permanent magnet is a small convenient magnetic field generating means. However, it is difficult to generate a large magnetic field of about 2 to 5 tesla using such a magnet. According to the reported results of experiments using an NdFeB-based permanent magnet and Gd as a magnetic material for magnetic refrigeration, the cooling temperature at room temperature is very low, 1.6° C., because the magnitude of the magnetic field is small. This greatly differs from the refrigerating ability of the conventional gas compression/expansion cycle.